This invention relates to controlling corrosion and, more particularly, to a method for inhibiting the corrosion of ferrous-based materials imbedded in concrete containing a chloride.
In the United States, billions of dollars have been spent in the construction of highways, freeways and their associated overpasses and bridges. One of the most important problems facing the nation is determining how to maintain the integrity of this system of roads and associated structures at an acceptable cost.
A problem which reduces the integrity of roadways, related structures and buildings located near sources of saltwater, is the corrosion of the contained reinforcing material by chloride-based deicers, seawater and other sources of chlorides. These substances continuously attack the reinforcing materials within concrete, causing the roadways and structures to degrade and ultimately fail prematurely. At a potential cost of billions of dollars, the nation is confronted with the task of repairing its highway system and other structures by removing the corroding reinforcing steel and replacing it with new reinforcing material. Yet, if the corroded steel is not replaced and corrosion is allowed to continue to critical stages, the road surfaces and structures may potentially fail catastrophically with associated human losses.
In an effort to avoid such failures and possibly to find a less expensive solution than replacing the reinforcing materials, much research has been conducted to learn how to stop the corrosion of steel imbedded in concrete. Among the publications reporting such work are the following: Mozer, Bianchi and Kesler, "Corrosion of Reinforcing Bars in Concrete," Journal of American Concrete Institute, August, 1965; Tremper, "Corrosion of Reinforcing Steel"; ASTM Special Technical Publication No. 169-A; Significance of Test and Properties of Concrete Making Materials; Spellman and Stratful, "Concrete Variables and Corrosion Testing," California Department of Public Works, Division of Highways, Materials and Research Department, Research Report No. M & R 635116; Gouda and Monfore, Journal of PCA Research and Development Laboratories, Ser. 1175, September, 1965. In addition to these publications, on August 14, 1992, DIALOG INFORMATION SERVICES published abstracts of publications which describe various tests and research projects directed to the problem of inhibiting the corrosion of steel imbedded in concrete. The abstracts describe, for example, using sodium nitrite, calcium nitrite, the lignosulphonates, and calcium magnesium acetate as corrosion inhibitors.
The published literature appears to indicate that concrete, alone, inhibits the corrosion of imbedded reinforcing steel. The corrosion may in some instances be chemical, but it is more commonly electrochemical in nature. The area of the steel where metal ions go into solution in an amount chemically equivalent to the reaction at cathodic regions is the anodic region. If the metal is iron, it goes into solution and forms ferrous ions, Fe.sup.++, plus two electrons, 2e.sup.-, to maintain an equilibrium of electrical charges. An equivalent quantity of hydrogen is plated out as a thin film at the adjacent surface regions of the metal known as the cathode. This thin film of hydrogen inhibits further corrosion of the iron surface. The anodic and cathodic reactions are summarized as follows: EQU At anode: Fe.fwdarw.Fe.sup.++ +2e.sup.- (1) EQU At cathode: 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 (2)
The reaction at the cathode regions is relatively slow in alkaline media because the concentration of hydrogen is very low. This reaction rate is increased, however, by the depolarizing action of dissolved oxygen according to the following reactions: EQU 2H.sup.+ +1/2O.sub.2 +2e.sup.31 .fwdarw.H.sub.2 O (3) EQU Fe+H.sub.2 O+1/2O.sub.2 .fwdarw.Fe(OH).sub.2 (4) EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.- (5)
The corrosion rate is proportional to the oxygen concentration, and the quantity of electricity flowing through the local galvanic cells is equivalent to the amount of metallic corrosion. With increasing anodic polarization, the overall corrosion of the metal diminishes. In ordinary conditions of reinforcing steel in concrete, where pH is high and the hydrogen ion concentration is low, and where there is essentially no oxygen supply, an anodic coating forms on the steel and stops the corrosion reaction. When chloride deicing salts are present, however, the protective iron oxide and hydrogen films are removed from the steel surface by forming soluble chloride compounds. The loss of these protective films exposes the iron or steel surface to further electrochemical attack.
Gouda and Monfore have stated, "Since areas that corrode are anodic, valuable information may be obtained on a macroscopic scale by forcing the whole metal to be anodic. This can be accomplished by applying an external voltage between the metal as an anode and an auxiliary electrode as cathode. Polarization current densities of from 1 to 1000 micro-amperes per cm.sup.2 are usually applied in such tests, presumably by approximating the values encountered in actual local cells."
Iron is above hydrogen in the electromotive force series, and corrodes readily when not imbedded in concrete and exposed to either oxygen or to hydrogen ions. Water in the open atmosphere may contain both hydrogen ions and dissolved oxygen, and thus may corrode steel.
The difficulty of developing an inhibitor to stop the corrosion of steel in concrete is evident from the amount of unsuccessful research which has so far been conducted. In the publication by Mozer, Bianchi and Kesler entitled "Corrosion of Reinforcing Bars in Concrete", the authors explain at page 927 that:
"Anodic inhibitors contain materials such as alkalies, phosphates and chromates which form either iron salts or a ferric oxide film on the anodic surface thus preventing ferrous ions from entering the solution. Such anodic inhibitors are effective only in high concentrations. If they are added in insufficient quantities, the corrosion reaction may be locally intensified. On the other hand, high concentrations may adversely affect the concrete. Therefore, the use of conventional anodic inhibitors cannot be recommended until more complete and fundamental investigations have been conducted."
In light of this statement, to avoid the local intensification of the corrosion reaction, the inhibitor should cover the entire surface of the steel and, furthermore, should not deleteriously affect the surrounding concrete.